Conventionally, a number of protective elements in a power supply circuit have been automated recovering bimetallic protectors or non-automated recovering elements using a meltable element such as a temperature fuse, a current fuse, etc., and also a number of combinations of a fuse, a protector, and a heating resistor have been widely used.
When a resistor is a main component, it is built into a cement resistor, and when a fuse is a main component, a meltable element and a resistor are built into a plate which is implemented on a printed circuit for commercial use.
These protective elements are used in interrupting and detecting an abnormal current, and also in energizing a resistor and forcibly interrupting a current.
A protective device represented by a common protector is set to operate automatically through changes in temperature and current in order to avoid the possibility that a part melts and becomes disconnected due to overheating caused by an abnormal ambient temperature, an excess current flow, etc.
For example, the conditions are set for protection against overheating in a case in which a temperature of 150° C. or above is attained, which is hazardous, for protection against overloading in a case in which a current of 20 A or greater is to be interrupted, etc. If these abnormal situations are temporary occurrences it is necessary for a protector to be an automated recovering unit.
On the other hand, an automated recovering protective element can continuously enter a hazardous state or proceed toward a worse state due to a fault with an external factor to a power supply in a power supply circuit, for example, due to an overload, a short circuit, or overheating caused by insufficient radiation.
The reuse of a protective circuit may not be realized if a non-automated recovering protective element such as a conventional fuse is operated as a protector for a countermeasure against the above-mentioned fault. In this case, a manually reset protector or a self-sustaining protector can be used.
However, when such a hazardous state is detected, advanced countermeasures can be taken to ensure safe operation if the hazardous state can be avoided by intentionally operating a protective element via an electronic circuit and software.
It is all the more necessary for an expensive system to be protected and for higher reliability to be achieved in stopping a function before a fault in an internal part occurs, in avoiding a hazardous state, and in realizing reuse.
An external operation thermal protector is appropriate for restoring a system to a state in which reuse can be realized after confirmation of security of the system by avoiding a hazardous state of the system when a protective element is intentionally operated as described above.
Generally, a PTC (positive temperature coefficient) element is used as a heating resistor that is available as a protector. PTC elements are roughly classified into ceramic PTC elements and polymer PTC elements. Although ceramic PTC elements are expensive, they are stable in shape against thermal change. Therefore, they are easily incorporated into a protector body as a part.
Since ceramic PTC elements are stable in shape as heating resistors regardless of thermal change, the heating resistor can be fixed and incorporated by a strong upward and downward push to effectively use thermal conductivity when it is incorporated into the protector body.
For example, as with U.S. Pat. No. 3,825,583, a bimetallic protector obtained as a combination of a bimetal and a heating resistor is proposed as an example of a conventional operation thermal protector. With the protector, a PTC element is caulked and crimped for assembly. That is, a ceramic PTC element is assumed in this case.
On the other hand, since most thermal protectors for protection against an excessive increase in temperature in a circuit with a voltage equal to or lower than a commercial supply voltage have a small necessary amount of current and have low-price circuit configurations, it is advantageous to use polymer PTC elements, which operates with low resistance, rather than ceramic PTC elements, as the former are less expensive than the latter.
Polymer PTC elements are made by dispersing conductive particles, for example, carbon particles, on an insulating synthetic resin, and the principles of their current interruption abilities are well known. Even if a current passes through the conductive path formed through conductive particles at a normal temperature, it causes volume expansion due to thermal expansion around the melting point of a synthetic resin at a high temperature, thereby disconnecting the electrical connections between the conductive particles, suddenly raising the inner resistance, and greatly decreasing the current.
Volume expansion due to a thermal effect as described above is important for the current interrupt operation of the polymer PTC element. If the volume expansion is restricted or if compressive expansion occurs on the body of the polymer PTC element due to a strong pressure when the current is interrupted, then localized current concentration occurs and a hot spot is generated.
Therefore, incorporating the polymer PTC element into a protector is not as easy as incorporating the ceramic PTC element, which can be incorporated anywhere a fixing process can be performed.